1. Field of the Invention
The invention relates to magnetic film layers, and more specifically to providing smooth barrier layers in magnetoresistive devices.
2. Brief Description of the Related Art
Magnetoresistive devices are known which take advantage of a magnetoresistive effect in electrically-conductive, multilayer structures containing ferromagnetic regions. The magnetoresistive effect is characterized by changes in electrical conductivity within the device due to the relative orientation of magnetic moments in the ferromagnetic regions. The magnetic orientation can be controlled by external fields, for example. Typically, the relative alignment of magnetic moments in the ferromagnetic regions can be parallel or antiparallel. Parallel alignment gives rise to one resistance in the device, whereas antiparallel alignment produces a different resistance. The different resistance values of the device can be translated into different logic signals.
Examples of magnetoresistive devices include magnetic memory cells, which typically are used in magnetic random access memory systems, and magnetic sensors, examples of which include disk drive read sensors in magnetic disk storage systems.
The typical structure used in magnetoresistive devices is a sandwich of two ferromagnetic layers separated by a non-magnetic layer. The magnetic orientation of one ferromagnetic layer is fixed to provide a reference, while the magnetic orientation of the other ferromagnetic layer remains free to be reoriented under the influence of external forces, such as magnetic bits on a storage disk, or an induced magnetic field controlled by a computer memory system. As an example of a memory system, magnetic memory cells are employed in magnetic random access memory (MRAM) devices to store information as the orientation of magnetic moments in a ferromagnetic region. Magnetic memory cells typically include a non-magnetic tunnel junction layer formed between two ferromagnetic layers. The two ferromagnetic layers include a pinned (fixed) ferromagnetic layer and a free ferromagnetic layer.
An example of such a memory cell 2 is illustrated by the magnetic tunnel junction (MTJ) memory cell of FIG. 1, in which two ferromagnetic layers, 4 and 6, are separated by a thin non-magnetic tunnel barrier layer 8. Memory cell 2 generally is formed over a substrate 10, such as a silicon substrate, for example. The memory cell may be separated from the substrate containing active devices by one or more insulating layers, such as tetra-ethyl-ortho-silicate (TEOS) layer 12. A protective cap layer 14 is provided on top of the memory cell. The pinned and free ferromagnetic layers typically are formed of one or more layers of NiFe or CoFe and the non-magnetic barrier layer 8 typically is formed of aluminum oxide (Al2O3) or Cu. Magnetic memory cells can hold stored information for a long time, without the need for a current supply or data refresh, and thus are non-volatile. MTJ cells can be used in non-volatile magnetic memory storage cells, which can replace conventional capacitive storage cells in DRAM memory devices, for example.
An amount of current allowed to tunnel across barrier 8 in MTJ cell 2 depends on the orientation of the magnetic moments of ferromagnetic layers 4 and 6. The tunneling current is higher when the magnetic moments are aligned parallel to one another, giving rise to a magnetic-tunneling effect. Configured for use as a memory storage cell, the MTJ is arranged so that the magnetic moment of pinned ferromagnetic layer 4 is fixed, hence “pinned,” while the magnetic orientation of free ferromagnetic layer 8 is established by an external field, thereby controlling the magnetoresistive state of the MTJ memory cell.
Magnetic sensors, known also as spin valves, based on a giant magnetoresistance (GMR) effect, have a structure similar to that of magnetic tunnel junction devices. Used as disk drive read heads, for example, the sensor structure 20 essentially includes four thin layers of material, as shown in FIG. 2. A free layer 22 is the sensing layer, commonly made of nickel, iron, or cobalt alloys. An orientation of magnetic moments rotates in free layer 22 in response to the magnetic patterns on a disk, for example, as the read head passes over the surface of the data bits to be read. Pinned layer 24, generally formed of a cobalt material, is held in a fixed magnetic orientation by an adjacent exchange layer 26. Spacer layer 28 is nonmagnetic, typically made from copper, and is disposed between the free and pinned layers. Exchange layer 26 is made of an “anti-ferromagnetic” material, typically constructed from iron and manganese, and fixes the pinned layer's magnetic orientation.
During read head operation, when electrons in the free layer become aligned with those of the pinned layer, a lower resistance is created in the entire head structure. When the head passes over a magnetic field of the opposite polarity (“1”), electrons in free layer 22 rotate so that they are not aligned with those of the pinned layer, causing an increase in the resistance of the overall sensor structure 20. The resistance changes are caused by changes to the spin characteristics of electrons in the free layer.
The manner of fabricating MTJ and GMR devices impacts their magnetic and magnetoresisitive properties. Surface and interface effects which may affect properties of the fabricated devices include interlayer coupling, Néel coupling, surface diffusion, interdiffusion at interfaces, and specular electron scattering at surfaces. In some GMR devices, it is possible to control these factors or to use them to manipulate the growth or improve post-growth processing of spin valves to improve their magnetic and magnetoresistive properties. Specular scattering is particularly important for achieving the largest possible GMR values. In MTJs, coupling between layers introduces an unwanted magnetic field bias threshold that must be overcome to switch magnetic moments from one orientation to another.
Magnetoresistive devices, based on MTJ and GMR structures, are being produced on a smaller and smaller physical scale, with nanoscale structures becoming feasible. At these smaller dimensions, effects of aberrant surface morphology become increasingly more significant. During typical magnetic element fabrication, such as MRAM element fabrication, which includes metal films grown by sputter deposition, evaporation, or epitaxy techniques, the film surfaces are not absolutely flat but instead exhibit surface or interface waviness. This waviness of the surfaces and/or interfaces of the ferromagnetic layers is the cause of magnetic coupling between the free ferromagnetic layer and the other ferromagnetic layers, such as the fixed layer. This is known as topological coupling or Néel coupling. Such coupling typically is undesirable in magnetic elements because it creates the magnetic field bias offset in the response of the free layer to an external magnetic field.
Since microscopic discrepancies in surface smoothness influence the magnetic behavior of the layers, improved manufacturing processes are required to produce film layers having small grain size and smooth, consistent surfaces, to minimize the extraneous dipoles that influence Néel coupling, and increase specular scattering.